Hybrid microcircuits are commonly used in the electronics industry. Typically, each of these circuits is on a ceramic or printed circuit board (PCB) substrate and has active and/or passive components on one side. The hybrid microcircuits are typically attached to a mother board (main PC Board) by leads or by surface mount terminals.
Many microcircuits need to be precisely tuned or adjusted in order to operate at the proper parameters. Typically, this tuning process involves changing the value of a resistor or a capacitor by physically removing a portion of the resistor or capacitor. In the prior art, one way of doing this was by sand trimming thick film or thin film resistor that was printed on the circuit. When a small jet of sand is impinged upon the resister at high velocity, the sand particles are brayed away selected portions of the resistor, thus changing the resistance value. In an active tuning situation, the parameters of the circuit are measured while the resister is being trimmed and the process is terminated when the circuit parameters reach the desired values. This is an example of a closed loop feed back trimming system. Circuits that operate at or near radio frequencies (RF) typically need to be shielded from spurious RF signals that may interfere with the operation of the circuit. This is typically performed by placing a metal enclosure over the circuitry to prevent any unwanted RF signals from either entering or leaving the enclosure.
The problem with the conventional art is that if the circuit is trimmed or adjusted prior to attachment of the RF shield, the performance of the circuitry is changed once the shield is attached. This is due to the fact that subtle interactions between the circuit, the shield and the surrounding RF environment cannot be predicted prior to the attachment of the shield. Thus, even in the best of situations, the trimming process is an approximation rather than an exact tuning. Clearly, once the shield is attached, it is impossible to trim the resister or capacitor because it is underneath the shield and in accessible to the trimming beam.
Many inventors have attempted to solve this problem by creating small windows in the RF shield to allow, for example, a laser beam to trim the circuit However, in certain situations even these small openings will effect the performance of the circuit and thus precise trimming is not possible.
Clearly, a solution to this classical problem would be desirable, that is a method of trimming the circuit after the shield is attached to allow precise and accurate adjustment of the circuit. This solution would also preclude the use of apertures in the shield.